1. Field of the Invention
The invention relates generally to electron spray aperture disks for use in electron microscopes and, more particularly, to disposable aperture disks for use in column liners in scanning electron microscopes.
2. Description of the Prior Art
The scanning electron microscope is a versatile and widely used tool of modern science. It can produce images over a wide range of magnifications and, in general, the magnification is far greater than that provided by the most advanced light microscopes.
In an electron microscope a beam of electrons is used instead of light as the source of illumination. The electron beam interacts with a specimen to yield a high resolution image of the specimen's surface.
The scanning electron microscope uses a focused beam of high energy electrons that systematically scans across the surface of a specimen. The interaction of the beam with the specimen produces a large number of signals at or near the specimen's surface. Low energy, or secondary electrons, are collected by a positively biased detector system. This electron signal is then converted to an electronic signal which is displayed on a cathode ray tube.
Structurally, the typical scanning electron microscope is composed of an electron gun assembly which produces a primary electron beam; electromagnetic lens and apertures, which focus the primary beam on the specimen; a vacuum system which allows free passage of the electron beam through a column without interference with air molecules; a specimen stage for holding the specimen in an optimum viewing position; and signal detection and display components, which permit the viewing of enlarged images of the specimen.
Surfaces of the several components exposed to the electron beam must be periodically cleaned and most electron microscopes include removable column liner tubes, through which the electrons pass, to simplify maintenance. The column liners pass through the bores of the magnetic lens and typically contain one or more removable metal aperture disks. These aperture disks have a small centrally located aperture. The aperture disks are located at critical positions within the column liner and serve to block off-axis beam electrons from reaching the specimen. Aperture disks, sometimes loosely called "spray apertures", are usually precision machined 3 mm platinum disks with very small central holes. They are placed in spring-loaded metal holders which are then slid into the column liner from one end. Such platinum aperture disks are expensive, but are capable of being cleaned and re-used. In order for these apertures to function properly they must also be able to be centered prior to insertion in the column liner, since once in the column liner they cannot be externally adjusted.